Protective packaging materials are commonly used to cushion a wide variety of products during shipping. These packaging materials exist in a wide variety of forms, including waste paper, embossed paper, foam beads or "peanuts" and expanded foams. Some of these materials are time-consuming to use, while others require a large amount of storage space, as well as disposal problems. Moreover, these forms of packaging materials do not always provide the cushioning needed when shipping and/or during other product handling conditions.
In seeking better protective packaging materials, various forms of air inflated cushions have been suggested. These cushions may be used to completely surround the article, to surround the end of an article and protect it from the outer container, and to separate articles from one another within an outer container.
Typically, inflatable packaging cushions are made from thermoplastic sheets which have been hermetically sealed around their periphery for retaining a fluid, such as air under pressure. An important criteria in using these cushions is that they not be punctured or otherwise deflated before the packaged article has safely reached its destination. In that regard, the films used to form these inflatable cushions take on a critical role. The films must exhibit high strength, high puncture resistance, and low gas or air permeability, and be capable of forming and maintaining a hermetic sheet-to-sheet seal. Examples of such inflatable cushions are disclosed in Farison, et al., U.S. Pat. No. 5,588,533; Pozzo, U.S. Pat. No. 5,620,096; and Pozzo, U.S. Pat. No. 5,803,263, all owned by the same assignee of the present application, the disclosures of which are incorporated herein by reference.
Presently, inflatable packaging cushions are often formed from multilayer films which are sealed together to form air pockets using radio frequency (Rf) energy. These films typically have an outer Rf active seal layer that includes an effective amount of an Rf active polymer such as high vinyl acetate ethylene/vinyl acetate (HVA-EVA) copolymer. By high vinyl acetate (HVA), it is meant that the ethylene/vinyl acetate (EVA) copolymer has a sufficiently high level of the polar vinyl acetate monomer to provide the copolymer with the ability to absorb an amount of Rf energy effective to make a seal. Effective amounts of vinyl acetate monomer in the copolymer include from about 12% to 28%, more particularly 18% to 28%, by weight of the copolymer. The HVA-EVA layer in the film thus acts as a receptor for the radio-frequency (Rf) energy needed to make the Rf seal that holds the two sheets of the film together.
Generally, an Rf active polymer is a polymer that efficiently absorbs Rf energy as a result of the polymer's chemical nature. An Rf active seal layer contains an amount of Rf active polymer effective to make an Rf seal when exposed to Rf energy. The Rf activity of a seal layer may be characterized by its dielectric loss factor, which is the composite of the dielectric loss factors of the constituents of the seal layer. The dielectric loss factor of a polymer is associated with the dipole moment about a carbon center. A polymer having a larger dipole moment is generally more likely to be active in an Rf energy field, i.e., more susceptible to excitation by Rf energy, and thus will generally have a higher dielectric loss factor.
Ethylene/butyl acrylate copolymer (EBA) and ethylene/vinyl acetate (EVA) may both be considered Rf active polymers if a sufficient amount of the butyl acrylate or vinyl acetate moieties are present. However, because EBA has a higher dipole moment than EVA, EBA can achieve a dielectric loss factor of about 0.2 with only 6 mole % butyl acrylate, whereas EVA requires 12 mole % vinyl acetate to achieve the same dielectric loss factor.
Rf activity of a polymer is also associated with the mass balance about a central carbon. A lower mass difference generally indicates less Rf activity. For example, ethylene/acrylic acid copolymer has a lower dielectric loss factor than EBA. This is in part because of the relatively low difference in atomic weight between the COOH [45] and the opposing hydrogen [1] of the ethylene/acrylic acid copolymer compared to the much greater difference between the COO(CH.sub.2).sub.3 CH.sub.3 group [101] opposing the hydrogen [1] of EBA.
The Rf activity of a polymer may also depend on the differential density about a central carbon, with a greater differential density indicating greater tendency for Rf activity. For example, polyvinyl chloride (PVC) has a chlorine atom [35.4] opposite the hydrogen atom [1] and therefore has a relatively high differential density. Therefore, although PVC has a lower mass imbalance about the main chain carbon than does EBA, PVC nevertheless is Rf active in part because of the large differential density.
Rf sealed inflatable cushion films also generally include a polyamide layer which provides significant wall strength to the package and serves as an air barrier enabling the cushions to retain air under pressure. Without sufficient barrier, the air will permeate from the high pressure package over time. One known inflatable packaging cushion from HVA-EVA copolymer which employs Rf sealing is constructed from a ten-ply, collapsed bubble. The package is produced from 10 mils of a blown film having the structural layers of HVA-EVA/EVA adhesive/polyamide/EVA adhesive/EVA/EVA/EVA adhesive/polyamide/EVA adhesive/EVA. The film is about 80% by weight HVA-EVA polymer.
There are many drawbacks to the use of Rf energy to seal together packaging cushion films. Firstly, the Rf active seal layer requires the presence of an effective amount of Rf active polymer (e.g., HVA-EVA). This requirement limits flexibility in designing an inflatable cushion packaging film and may cause an overall film thickness greater than that needed merely for strength and barrier properties. For example, inflatable packaging cushions films that incorporate an Rf seal layer of HVA-EVA must be at least about 10 mils thick to seal properly using Rf energy.
Further, Rf active polymers are generally lower in melting point and lower in tensile strength. The relatively low melting point of the Rf active polymers limits the temperature range for end uses because at elevated temperatures many Rf active seal layers become soft enough to rupture the Rf seals. Additionally, since one of the primary uses for inflated cushion packages is to protect expensive objects during shipment, the low tensile strength of the Rf active seal layers and polymers may often be inadequate for protection of heavy objects due to seal rupture during drop testing.
As a consequence, many Rf sealed films must be subjected to an ionizing radiation step, i.e., electron beam or gamma radiation, to crosslink the polymers to improve seal strength. This post irradiation treatment adds additional expense, time, and complexity to the manufacture of these films. However, without the post irradiation treatment, Rf seals formed in films such as those employing HVA-EVA are known to fail above 135.degree. F., and more particularly are known to fail the International Safe Transit Association Standards for Elevated Temperature Testing.
Another disadvantage of Rf seals is that Rf sealing requires relatively complex tooling and expensive electronic equipment. A discussion of Rf sealing and the dielectric loss factor for various polymers is in Encyclopedia of Polymer Science and Engineering, Volume 5 (1993), which is incorporated herein by reference.
There exists a need for thermoplastic films which may be directly heat sealed together in forming inflatable packaging cushions, thereby avoiding the need for radio frequency sealing and the drawbacks associated therewith. Preferably, such films will be stronger, more heat resistant, and also will avoid the need for any post sealing irradiation or other processing steps, thereby simplifying the cushion manufacturing process.